1. Field of the Invention
The present invention relates to an image forming apparatus employing a contact electrophotographic developing method, and more particularly, to an image forming apparatus that forms a uniform nip between a photosensitive member and a transfer device.
2. Description of the Related Art
An electrophotographic developing method is used in a wide range of image forming apparatuses such as copiers, laser printers, LED print head printers, facsimile machines, etc. The stages of the electrophotographic developing method include electric charging, light exposure, development and fixing.
FIG. 1 is a view showing an image forming process according to a conventional electrophotographic processor. Referring to FIG. 1, an electric-charge roller 44 electrically charges the surface of a photosensitive drum 40 with a uniform charge. After being electrically charged, the surface of the photosensitive drum 40 receives electric signals to form an image from an exposure unit (not shown), which exposes the photosensitive drum to light. Accordingly, an electrostatic latent image is formed on the photosensitive drum 40. The electrostatic latent image is developed by toner that is fed from a developer unit 48, and accordingly, turned to a visible image.
Meanwhile, a recording medium 52 stacked in a recording medium feed cassette 50 is picked up by a pickup roller 54, and conveyed between the photosensitive drum 40 and a transfer roller 56 by the rotation of conveyance rollers 55.
Next, by the electrophotographic interaction between the photosensitive drum 40 and the transfer roller 56, the visible image on the photosensitive drum 40 is transferred onto the surface of the recording medium 52. More specifically, due to the negative electric charge of the toner transferred onto the photosensitive drum 40, when positive electric charge is applied to the transfer roller 56, which is in contact with the photosensitive drum 40 with the recording medium 52 being placed therebetween, the toner image on the surface of the negative photosensitive drum 40 is transferred onto the recording medium 52 of the positive electric charge.
The quality of the image transferred onto the recording medium 52 depends on the voltage transferred onto the transfer roller 56, and a uniformity of a nip between the photosensitive drum 40 and the transfer roller 56 in contact with each other. Here, the ‘nip’ is substantially a line formed in a lengthwise direction when the photosensitive drum 40 and the transfer roller 56 contact each other.
After the image is formed on the recording medium 52, the recording medium 52 is passed between a heating roller 10 and a pressing roller 30 to fix the image to the recording medium 52 by a high degree of heat from the heating roller 10 and pressure from the pressing roller 30. After the image is fixed, the recording medium 52 is discharged outside the printer body by the rotation of an upper discharge roller 38 and a lower discharge roller 36, and then piled on a document tray 58 disposed outside of the body. After the toner transfer, there remains residual toner and an electrostatic latent image on the surface of the photosensitive drum 40, which are eliminated by a cleaner 42 and an electric-charge elimination lamp (not shown).
FIG. 2 is a side view showing the conventional transfer roller 56. Referring to FIG. 2, the transfer roller 56 has a cylindrical conductive rubber portion 57 having a length L, and a center shaft 60 serving as a center of rotation of the cylindrical conductive rubber portion 57. The cylindrical conductive rubber portion 57 is formed such that the diameters a1, a2 and a3 in an axial direction are identical to each other. At opposing ends of the cylindrical conductive rubber portion 57, springs 61 are disposed. One end of each spring 61 is supported on a frame 62 of the printer body. Accordingly, by a predetermined pressure of the springs 61 in an upward direction (i.e., toward the photosensitive drum 40), a nip is formed between the transfer roller 56 and the photosensitive drum 40.
FIG. 3 is a front view showing the transfer roller 56 and the photosensitive drum 40 in contact with each other, and forming the nip therebetween. Referring to FIG. 3, the transfer roller 56 is disposed under the photosensitive drum 40, and the nip is formed when the cylindrical conductive rubber portion 57 of the transfer roller 56 contacts the outer circumference of the photosensitive drum 40.
Here, the transfer roller 56 has the same stress distribution as a beam with the opposing ends being supported. Accordingly, due to a varying moment value, an intermediate portion of the conductive rubber droops. Since the reaction force at both ends of the transfer roller 56 is the same, the maximum value of the droop (t) can be found at a point distanced from one end of the cylindrical conductive rubber portion 57 by a length of L/2.
If the nip is formed between the photosensitive drum 40 and the transfer roller 56 with the droop occurring at the intermediate point of the transfer roller 56, the pressing force of the intermediate point of the cylindrical conductive rubber portion 57 against the photosensitive drum 40 is relatively weaker than the pressing force at both ends of the conductive rubber portion 57. Accordingly, toner transferability from the photosensitive drum 40 to the recording medium 52 at the intermediate point of the cylindrical conductive rubber portion 57 is deteriorated as compared to both ends. As a result, when the recording medium 52 passes through the transfer roller 56 and the photosensitive drum 40, image density at the center of the recording medium 52 is weaker than at the opposing ends.
Furthermore, when placed in a relatively high temperature for a long time, there sometimes occurs a migration of the substances of a low molecular mass inside the transfer roller 56 to the contact area between the transfer roller 56 and the photosensitive drum 40. This causes a horizontal band to appear in the vicinity of the contact area of the photosensitive drum 40. In order to minimize such migration, a special rubber resin must be added while coating or tubbing must be performed on the outer layer of the transfer roller 56. As a result, manufacturing costs increase.